Control method for multi-mode continuous casting and rolling production line strip steel surface slag inclusion defect

Abstract

The invention provides a control method for a multi-mode continuous casting and rolling production line strip steel surface slag inclusion defect, and belongs to the field of steel preparation. The method comprises the following steps: carrying out desulfurization treatment on blast furnace molten iron to obtain desulfurized molten iron with set sulfur content; the desulfurized molten iron is subjected to converter smelting, and converter molten steel with the set oxygen content is obtained; the converter molten steel is subjected to LF refining, and the molten steel is subjected to calcium treatment and soft blowing treatment for a set time before LF refining is carried out, so that refined molten steel with the set calcium content is obtained; and the refined molten steel is subjected to continuous casting, the pulling speed of a casting machine is increased in a mode dividing mode at the pulling speed rising stage of continuous casting, and a continuous casting blank is obtained. Through narrow-interval control on smelting molten steel end point oxygen, refining top slag oxidability is reduced, the outbound calcium content and soft blowing time are guaranteed, and through a staged acceleration strategy, the liquid level fluctuation is within + / -3mm, and the multi-mode continuous casting and rolling production line strip steel surface inclusion defect is successfully controlled at the optimal level.

Description

Technical Field

[0001] This application relates to the technical field of steel preparation, and particularly to a method for controlling slag inclusion defects on the surface of strip steel in a multi-mode continuous casting and rolling production line. Background Art

[0002] During the multi-mode thin slab continuous casting and rolling process, when the casting speed of the caster changes with different mode switches, or when the submerged nozzle is blocked due to poor cleanliness of the molten steel, it will cause large fluctuations in the mold flow field, resulting in the entrainment of mold powder, and finally forming slag inclusion defects on the surface of the strip steel. This defect has become a major problem in the surface quality control of strip steel in multi-mode continuous casting and rolling production lines.

[0003] However, in the prior art, the control of slag inclusion defects on the surface of strip steel is almost all aimed at conventional production lines, that is, by increasing the viscosity of the mold powder and the interfacial tension between steel and slag to reduce slag entrainment. However, these measures are not suitable for the production characteristics of multi-mode production lines. As the world's first multi-mode continuous casting and rolling production line, the slag inclusion problem of MCCR has not been effectively solved. Summary of the Invention

[0004] This application provides a method for controlling slag inclusion defects on the surface of strip steel in a multi-mode continuous casting and rolling production line to solve the following technical problems: how to reduce slag inclusion defects on the surface of strip steel in a multi-mode continuous casting and rolling production line.

[0005] An embodiment of this application provides a method for controlling slag inclusion defects on the surface of strip steel in a multi-mode continuous casting and rolling production line. The method includes:

[0006] Desulfurizing blast furnace hot metal to obtain desulfurized hot metal with a set sulfur content;

[0007] Smelting the desulfurized hot metal in a converter to obtain converter molten steel with a set oxygen content;

[0008] Refining the converter molten steel by LF, and performing calcium treatment and soft blowing treatment for a set time on the molten steel before the LF refining station exits to obtain refined molten steel with a set calcium content; and

[0009] Continuous casting the refined molten steel, and performing a sub-mode increase in the casting speed of the caster during the up-tension speed stage of the continuous casting to control the fluctuation range of the mold liquid level within ±3 mm to obtain a continuous casting billet.

[0010] Optionally, in parts per million concentration, the set sulfur content ≤ 20 ppm.

[0011] Optionally, the desulfurization treatment adopts a deep desulfurization mode, and the slag skimming bright surface of the desulfurization treatment > 90%.

[0012] Optionally, in parts per million concentration, the set oxygen content is 250 ppm to 500 ppm.

[0013] Optionally, the set time is 10 min to 12 min.

[0014] Optionally, in parts per million concentration, the set calcium content is 20 ppm to 30 ppm.

[0015] Optionally, by mass fraction, the sum of the contents of FeO + MnO in the top slag of the LF refining is < 1%, and the Als / Alt ratio in the top slag of the LF refining is > 0.91.

[0016] Optionally, the sub-mode for increasing the casting speed of the casting machine satisfies the following relationship:

[0017] In the starting casting stage, at a step of 1.5 m / min, without holding, the speed is increased to 3.5 m / min, and the single-billet mode is executed;

[0018] At a step of 0.2 m / min, maintaining a stable state for 3 min, the speed is increased to 4.3 m / min, and the semi-endless mode is executed;

[0019] At a step of 0.2 m / min, maintaining a stable state for 3 min, the speed is increased to the target casting speed, and the endless mode is executed.

[0020] Optionally, the continuous casting of the refined molten steel includes:

[0021] Injecting the refined molten steel from the ladle into the tundish; the tundish is protected by an alkaline covering agent, and the basicity of the covering agent is ≥ 2;

[0022] Injecting the refined molten steel from the tundish into the mold for continuous casting.

[0023] Optionally, the surface of the refined molten steel in the mold is covered with a powder, the basicity of the powder is 0.95 to 1.36, and the viscosity of the powder at 1300 °C is 0.7 Pa·S to 1.0 Pa·S.

[0024] The above technical solutions provided by the embodiments of the present application have the following advantages compared with the prior art:

[0025] The embodiment of the present application provides a method for controlling slag inclusion defects on the surface of strip steel in a multi-mode continuous casting and rolling production line. By narrowly controlling the oxygen content at the end of smelted molten steel, reducing the oxidability of the refining top slag, ensuring the calcium content at the tapping, the soft blowing time, and through a staged speed-up strategy, the liquid level fluctuation is controlled within ±3 mm, successfully controlling the slag inclusion defects on the surface of strip steel in the multi-mode continuous casting and rolling production line at the optimal level. The number of strip steels with quality defects is reduced, the product downgrading loss caused by slag inclusion defects is decreased. At the same time, the cleanliness of the molten steel is ensured, the occurrence of casting problems is avoided, and the output is increased. Brief Description of the Drawings

[0026] The drawings herein are incorporated into the specification and form a part of this specification, showing the embodiments in line with the present application, and are used together with the specification to explain the principles of the present application.

[0027] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0028] Figure 1 It is a schematic flow chart of a method for controlling slag inclusion defects on the surface of strip steel in a multi-mode continuous casting and rolling production line provided by the embodiment of the present application. Detailed Embodiments

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present application with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present application.

[0030] The various embodiments of the present application may exist in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity, and should not be construed as a rigid limitation on the scope of the present application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single values within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within that range, such as 1, 2, 3, 4, 5, and 6, regardless of what the range is. Additionally, whenever a numerical range is indicated herein, it means including any cited number (fraction or integer) within the indicated range.

[0031] In addition, in the description of the specification of the present application, terms such as "including" and "comprising" mean "including but not limited to". In this text, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this text, "and / or" describes the association relationship of associated objects and indicates that three relationships can exist. For example, A and / or B can mean: A exists alone, A and B exist simultaneously, and B exists alone. Where A and B can be singular or plural. In this text, "at least one" means one or more, and "a plurality" means two or more. "At least one kind", "at least one of the following (items)" or similar expressions refer to any combination of these items, including any combination of single item (s) or plural items (s). For example, "at least one of a, b, or c", or "at least one of a, b, and c" can both mean: a, b, c, a - b (that is, a and b), a - c, b - c, or a - b - c, where a, b, and c can be single or multiple respectively. The "parts representation method" such as weight parts and mass parts represents the proportional relationship between each component. In the proportional relationships involved in this text, the parameters that need to be described by proportion should be understood as the antecedents of the proportion formula in the order of description, and the proportional numbers should be understood as the consequents of the proportion formula. For example, if the mass ratio of substance A, substance B, and substance C is 1:2:3, then substance A, substance B, and substance C should correspond one by one with the proportional numbers in the proportion formula in the order of description, that is, the mass of substance A: the mass of substance B: the mass of substance C = 1:2:3.

[0032] Unless otherwise specifically stated, various raw materials, reagents, instruments, and equipment used in the present application can be obtained through market purchase or can be prepared by existing methods.

[0033] Figure 1 It is a schematic flow chart of a method for controlling slag inclusion defects on the surface of strip steel in a multi - mode continuous casting and rolling production line provided for the embodiments of the present application.

[0034] As Figure 1 shown, the present application provides a method for controlling slag inclusion defects on the surface of strip steel in a multi - mode continuous casting and rolling production line, and the method includes:

[0035] S1. Desulfurize the blast furnace hot metal to obtain desulfurized hot metal with a set sulfur content;

[0036] In some embodiments, in terms of parts per million concentration, the set sulfur content ≤ 20 ppm.

[0037] In some embodiments, the desulfurization treatment adopts a deep desulfurization mode, and the slag - skimming bright surface of the desulfurization treatment > 90%.

[0038] When the sulfur content is less than 20 ppm, the desulfurization pressure of the LF furnace can be relieved, the LF treatment cycle and the number of inclusions can be reduced, the cleanliness of the molten steel can be improved, and the defect of calcium aluminate inclusions in the strip steel can be avoided. Exemplarily, the slag removal bright surface of the desulfurization treatment can be 91%, 92%, 93%, 94%, 95%, 96%, etc. The sulfur content in the desulfurized hot metal can be 5 ppm, 7 ppm, 10 ppm, 15 ppm, 18 ppm, 20 ppm, etc.

[0039] S2. Smelt the desulfurized hot metal in a converter to obtain converter molten steel with a set oxygen content;

[0040] In some embodiments, in parts per million concentration, the set oxygen content is 250 ppm to 500 ppm.

[0041] When the oxygen content is less than 250 ppm, it is necessary to control the carbon-oxygen product to be very low, which cannot be achieved due to technical bottlenecks and cost considerations; when the oxygen content is higher than 500 ppm, the oxygen in the molten steel will form endogenous oxide inclusions (such as alumina), which will cause the clogging of the submerged entry nozzle and product inclusion defects. Exemplarily, the oxygen content of the converter molten steel is 250 ppm, 300 ppm, 350 ppm, 400 ppm, 450 ppm, 500 ppm, etc.

[0042] S3. Refine the converter molten steel by LF, and perform calcium treatment and soft blowing treatment for a set time before the LF refining station exits to obtain refined molten steel with a set calcium content; and

[0043] In some embodiments, by mass fraction, the sum of the contents of FeO + MnO in the top slag of the LF refining is <1%, and the Als / Alt ratio in the top slag of the LF refining is >0.91.

[0044] In some embodiments, the set time is 10 min to 12 min.

[0045] In some embodiments, in parts per million concentration, the set calcium content is 20 ppm to 30 ppm.

[0046] During the LF refining process, the chemical composition of the top slag is crucial for the refining effect. The ratio of Als (acid-soluble aluminum) to Alt (total aluminum, including acid-soluble aluminum and combined aluminum) is an important parameter. By quickly creating a white slag and controlling the oxidizing property of the top slag, the mass fraction of FeO + MnO in the top slag is < 1%, and the Als / Alt ratio in the top slag is greater than 0.91. When FeO + MnO in the top slag ≥ 1%, the oxygen in the slag will react with the molten steel to form endogenous oxide inclusions (such as alumina), and when the Als / Alt ratio is less than 0.91, it indicates that the alumina inclusions have not been fully calcium-treated. Exemplarily, the sum of the contents of FeO + MnO in the top slag of LF refining can be 0.1%, 0.2%, 0.3%, 0.5%, 0.7%, 0.9%, 0.98%, etc., and the Als / Alt ratio in the top slag can be 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, etc.

[0047] By adjusting the addition amount of the calcium wire, the calcium content in the molten steel at the LF tapping is controlled between 20 ppm and 30 ppm. At this time, the alumina inclusions in the molten steel are fully calcium-treated and float up and are absorbed by the top slag, avoiding the problem of clogging of the submerged entry nozzle during continuous casting. Exemplarily, the calcium content in the refined molten steel can be 20 ppm, 22 ppm, 24 ppm, 26 ppm, 28 ppm, 30 ppm, etc.

[0048] In the embodiment of the present application, soft blowing treatment is performed on the molten steel about to be tapped, that is, argon bottom blowing in the ladle. By controlling the argon flow rate, the top slag layer vibrates slightly without breaking. Exemplarily, the soft blowing time of the molten steel can be 10 min, 10.5 min, 11 min, 11.5 min, 12 min, etc.

[0049] S4. Continuously cast the refined molten steel, and perform a split-mode increase in the casting speed of the casting machine during the stretching speed stage of the continuous casting to control the fluctuation range of the mold liquid level to ±3 mm, and obtain a continuous casting billet.

[0050] It should be noted that the qualified range of the liquid level fluctuation of the high-speed casting machine is within ±3 mm. Within this range, the flow field in the mold is stable and it is not easy to generate slag inclusion defects. The liquid level detection method adopted by the multi-mode continuous casting and rolling production line is radioactive source detection; during the rolling process of the slab, a strip surface detector is used to monitor the surface quality of the strip.

[0051] In some embodiments, the continuously casting the refined molten steel includes:

[0052] Inject the refined molten steel from the ladle into the tundish; the tundish is protected by an alkaline covering agent, and the basicity of the covering agent ≥ 2;

[0053] Inject the refined molten steel from the tundish into the mold to perform continuous casting.

[0054] During the casting process, protective casting measures are taken to prevent the molten steel from contacting the air and oxidizing. The tundish is protected by an alkaline covering flux, and the basicity of the covering flux is ≥2. A high-basicity covering flux is an important guarantee for absorbing inclusions in the continuous casting tundish. Theory and practice have proved that the basicity of the covering flux must be ≥2 to meet the characteristics of absorbing inclusions. Exemplarily, the basicity of the covering flux can be 2, 2.2, 2.5, 2.8, 3, 3.2, etc.

[0055] In some embodiments, the surface of the refined molten steel in the mold is covered with a protective slag. The basicity of the protective slag is 0.95 - 1.36, and the viscosity of the protective slag at 1300°C is 0.7 Pa·S - 1.0 Pa·S.

[0056] The surface of the molten steel in the mold is covered with a protective slag. The protective slag forms a slag film between the mold wall and the solidified shell, playing a lubricating role, reducing the drawing resistance, and preventing the adhesion of the solidified shell to the copper plate. At the same time, when the viscosity of the protective slag at 1300°C is 0.7 Pa·S - 1.0 Pa·S, it can not only ensure good fluidity but also avoid the liquid slag layer being too thick or too thin due to too fast a melting speed. The appropriate thickness of the liquid slag layer helps to reduce the generation of longitudinal cracks on the surface of the slab. Exemplarily, the basicity of the protective slag can be 0.95, 1.00, 1.05, 1.10, 1.20, 1.30, 1.36, etc., and the viscosity of the protective slag at 1300°C can be 0.7 Pa·S, 0.75 Pa·S, 0.80 Pa·S, 0.85 Pa·S, 0.90 Pa·S, 0.95 Pa·S, 1.0 Pa·S, etc.

[0057] In some embodiments, the casting machine drawing speed in different modes meets the following relationship:

[0058] At the starting casting stage, with a step of 1.5 m / min, without holding, the speed increases to 3.5 m / min, and the single-billet mode is executed;

[0059] With a step of 0.2 m / min, maintaining a stable state for 3 minutes, the speed increases to 4.3 m / min, and the semi-endless mode is executed;

[0060] With a step of 0.2 m / min, maintaining a stable state for 3 minutes, the speed increases to the target drawing speed, and the endless mode is executed.

[0061] The strategy for increasing the casting machine drawing speed in different modes provided by the embodiments of the present application fully considers the changes in the drawing speed and cooling intensity under different production modes.

[0062] In the starting casting stage, the casting speed in the single-billet mode is low. The cooling intensity of the mold and secondary cooling is in the initial rising stage, with low cooling intensity. To avoid uneven solidification of the billet shell and abnormal liquid level, a small step increase in speed is required and it does not need to be maintained. The semi-endless mode is in the middle stage of increasing the casting speed, and a faster step increase in speed is needed. However, since it is still in the non-steady state stage of liquid level fluctuation at this time, each speed increase needs to maintain a stable state for 3 minutes. In the endless mode, the mill has completed threading, and the caster needs to ensure stable high-speed casting to achieve casting-rolling matching. At this time, the semi-endless speed increase strategy should be maintained, and the rate of 0.2 m / min should be continued and maintained for 3 minutes until the target casting speed is reached.

[0063] Such a speed increase strategy for different modes fully matches the production characteristics of multi-mode continuous casting and rolling, and can maximize the avoidance of abnormal liquid level fluctuations caused during the speed increase, providing guarantee for the strip quality.

[0064] In summary, the control method for slag inclusion defects on the surface of strip steel in a multi-mode continuous casting and rolling production line provided by the embodiment of the present application has the following significant advantages:

[0065] (1) High-efficiency desulfurization and pure molten steel: The blast furnace hot metal is treated in the deep desulfurization mode, and the sulfur content can be reduced to below 20 ppm, significantly reducing the desulfurization pressure of the LF furnace, shortening the LF treatment cycle, reducing the number of inclusions, and improving the cleanliness of the molten steel. At the same time, the slag removal bright surface during desulfurization treatment is high (>90%), further ensuring the desulfurization effect and reducing the adverse impact of sulfur on the quality of molten steel.

[0066] (2) Precise control of oxygen content: The oxygen content of the molten steel after converter smelting is controlled within a narrow range of 250 ppm to 500 ppm, which not only avoids the technical bottlenecks and cost problems caused by too low oxygen content, but also prevents the generation of endogenous oxide inclusions due to too high oxygen content, ensuring the quality of the molten steel.

[0067] (3) Optimization of the LF refining process: By quickly making white slag, controlling the oxidizability of the top slag (FeO + MnO content < 1%), and maintaining the Als / Alt ratio > 0.91, the generation of inclusions such as alumina is effectively reduced, improving the cleanliness and castability of the molten steel. At the same time, precisely controlling the calcium content of the molten steel when leaving the LF (20 ppm to 30 ppm) enables the alumina inclusions to be fully calcium-treated and float to the top slag, avoiding the clogging problem of the submerged entry nozzle in continuous casting. In addition, appropriate soft blowing treatment (10 min to 12 min) ensures the full floating and removal of inclusions in the molten steel, further improving the quality of the molten steel.

[0068] (4) Stable continuous casting process: An alkaline covering agent (alkalinity ≥ 2) was used to protect the molten steel in the tundish, effectively absorbing inclusions and preventing secondary oxidation of the molten steel. A powder flux with appropriate alkalinity and viscosity was used in the mold to ensure good lubrication effect and moderate thickness of the liquid slag layer, reducing the generation of longitudinal cracks on the surface of the slab. At the same time, a strategy of increasing the casting speed in different modes was implemented. By adjusting the speed increase and holding time at different stages, the fluctuation range of the mold liquid level was successfully controlled within ±3 mm, providing guarantee for the strip quality.

[0069] (5) Improving product quality and output: Through the above comprehensive control measures, the inclusion defects on the surface of the strip in the multi-mode continuous casting and rolling production line were successfully controlled at the optimal level, reducing the number of defective strips with quality defects. The downgrading losses of products caused by slag inclusion defects were reduced, improving the market competitiveness of products and the economic benefits of the enterprise. At the same time, the cleanliness of the molten steel and the stability of the casting process were ensured, improving production efficiency and output.

[0070] The following will further elaborate on this application in combination with specific embodiments. It should be understood that these embodiments are only used to illustrate this application and not to limit the scope of this application. For the experimental methods without specific conditions noted in the following embodiments, they are usually determined according to industry standards. If there is no corresponding industry standard, they are carried out according to general international standards, conventional conditions, or the conditions recommended by the manufacturer.

[0071] Example 1

[0072] This embodiment provides a method for controlling the slag inclusion defects on the surface of the strip in a multi-mode continuous casting and rolling production line. The multi-mode production line smelts the common carbon Q235B variety, and the furnace number is A001. The method includes the following steps:

[0073] Step 1: The hot metal transferred from the blast furnace to the steel mill is subjected to KR desulfurization treatment, implementing the deep desulfurization mode, with the slag skimming surface > 90%, and the sulfur content after desulfurization is 18 ppm.

[0074] Step 2: The qualified hot metal after KR desulfurization is poured into the converter for smelting. Calculate the carbon-oxygen product and control the mass fraction of oxygen in the molten steel after the converter at 400 ppm.

[0075] Step 3: The qualified molten steel is subjected to LF refining. By quickly making a white slag, control the oxidability of the top slag. The mass fraction of FeO + MnO in the top slag is 0.71%, and the ratio of Als / Alt in the top slag is 0.93. By adjusting the addition amount of calcium wire, control the calcium content of the molten steel when LF tapping within 20 ppm. Soft blowing treatment is carried out on the molten steel about to tap, that is, bottom blowing argon in the ladle. By controlling the argon flow rate, make the top slag layer vibrate slightly without breaking, and the soft blowing time is 10 min.

[0076] Step 4: Pour the refined molten steel from the ladle into the tundish, and then pour the molten steel from the tundish into the mold for continuous casting to obtain a continuous casting billet. During the casting process, take protective casting measures to prevent the molten steel from contacting the air and oxidizing. The tundish is protected with an alkaline covering agent, and the basicity of the covering agent is 2.2. The surface of the molten steel in the mold is covered with a flux, the basicity of the flux is 1.1, and the viscosity of the flux at 1300 °C is 0.9 Pa·S.

[0077] Step 5: In the stage of increasing the casting speed, implement a strategy of increasing the casting speed of the casting machine in different modes to control the fluctuation range of the mold liquid level. Specifically, at the starting stage of casting, increase the speed in steps of 1.5 m / min to 3.5 m / min without holding, and implement the single-billet mode at this stage. Then, implement a speed of 0.2 m / min and hold for 3 min to increase the speed to 4.3 m / min, and implement the semi-endless mode at this stage. After that, still implement a speed of 0.2 m / min and hold for 3 min to increase the speed to the target casting speed, and implement the endless mode at this stage.

[0078] In this heat, there is a slight problem of nozzle clogging of the submerged entry nozzle, and a small amount of slag inclusions appear on the surface of the strip steel.

[0079] Example 2

[0080] This example is modified as follows based on the disclosure of Example 1:

[0081] The multi-mode production line smelts the common carbon Q235B variety, with the heat number A002. The sulfur content after KR desulfurization is 19 ppm, the oxygen content after the furnace is 420 ppm, the top slag (FeO + MnO) is 0.82%, Als / Alt = 0.95, the calcium content at the tapping is 25 ppm, the soft blowing time is 11 min, the basicity of the tundish covering agent is 2.2, the viscosity of the flux is 0.9 Pa·S, and the strategy of increasing the casting speed in different modes is implemented.

[0082] In this heat, there is no problem of nozzle clogging of the submerged entry nozzle, and the surface quality of the strip steel is good.

[0083] Example 3

[0084] This example is modified as follows based on the disclosure of Example 1:

[0085] The multi-mode production line smelts the common carbon Q235B variety, with the heat number A003. The sulfur content after KR desulfurization is 18 ppm, the oxygen content after the furnace is 440 ppm, the top slag (FeO + MnO) is 0.6%, Als / Alt = 0.94, the calcium content at the tapping is 30 ppm, the soft blowing time is 12 min, the basicity of the tundish covering agent is 2.2, the viscosity of the flux is 0.9 Pa·S, and the strategy of increasing the casting speed in different modes is implemented.

[0086] In this heat, there is no problem of nozzle clogging of the submerged entry nozzle, but there is slight rod flushing, and the surface quality of the strip steel is good.

[0087] As can be seen from Examples 1 to 3, the surface quality of the strip steel in Examples 1 to 3 is good. However, in Example 1, when the calcium content at the furnace outlet is low and the soft blowing time is insufficient, the problem of nozzle blocking is likely to occur, and a small amount of slag inclusion defects are likely to appear on the strip steel surface; in Example 3, when the calcium content at the furnace outlet is on the high side and the soft blowing time is sufficient, the problem of nozzle erosion will occur, affecting the service life of the stopper rod; when other conditions remain unchanged and the calcium content at the furnace outlet and the soft blowing time are controlled within a certain range, the liquid level fluctuation is normal and the surface quality of the strip steel is good.

[0088] Comparative Example 1

[0089] The following modifications are made to this comparative example based on the disclosure of Example 1:

[0090] The multi-mode production line smelts the common carbon Q235B variety. The heat number is A004, the sulfur content after KR desulfurization is 23 ppm, the oxygen content after the furnace is 470 ppm, the top slag (FeO + MnO) is 1.02%, Als / Alt = 0.91, the calcium content at the furnace outlet is 15 ppm, the soft blowing time is 12 min, the basicity of the tundish covering agent is 2.2, the viscosity of the mold powder is 0.9 Pa·S, and the split-mode speed increase strategy is implemented.

[0091] In this heat, a serious problem of nozzle blocking occurred in the submerged entry nozzle. The nozzle blocking caused the stopper rod to continuously rise, resulting in the casting machine having to take speed reduction control measures. Correspondingly, slag inclusion defects continuously appeared on the strip steel surface in this heat. The longest defect length exceeded 1 m, and the number of defects exceeded 20. Compared with the normal control range, the calcium content at the furnace outlet was low and the oxidability of the top slag was strong, resulting in continuous generation of inclusions in the molten steel, which could not be fully modified and removed. Eventually, they adhered to the submerged entry nozzle, causing the nozzle blocking problem. The nozzle blocking caused a large fluctuation in the mold liquid level, and finally a batch of mold powder was involved in the solidified shell, forming slag inclusion defects on the strip steel surface.

[0092] Comparative Example 2

[0093] The following modifications are made to this comparative example based on the disclosure of Example 1:

[0094] The multi-mode production line smelts the common carbon Q235B variety. The heat number is A005, the sulfur content after KR desulfurization is 16 ppm, the oxygen content after the furnace is 450 ppm, the top slag (FeO + MnO) is 0.63%, Als / Alt = 0.97, the calcium content at the furnace outlet is 28 ppm, the soft blowing time is 11 min, the basicity of the tundish covering agent is 2.2, the viscosity of the mold powder is 0.9 Pa·S, and the split-mode speed increase strategy is not implemented, that is, the single-slab, semi-endless, and endless modes all increase the speed at a step of 0.2 m / min.

[0095] In this heat, slag inclusions appeared in multiple places on the strip surface in the single-slab mode, while no surface defects occurred in the semi-endless and endless modes. This shows that for a multi-mode production line, different stretching and speed strategies must be implemented according to different production modes. Otherwise, abnormal liquid level fluctuations will be caused by the speed differences in different modes, ultimately leading to the appearance of slag inclusion defects on the strip.

[0096] Comparative Example 3

[0097] This comparative example is modified as follows based on the disclosure of Example 1:

[0098] The multi-mode production line smelts the common carbon Q235B variety. The heat number is A006. The sulfur content after KR desulfurization is 31 ppm, the oxygen content after the furnace is 510 ppm, the top slag (FeO + MnO) is 1.3%, Als / Alt = 0.83, the calcium content at the station is 25 ppm, the soft blowing time is 12 min, the basicity of the tundish covering agent is 2.2, the viscosity of the mold powder is 0.9 Pa·S, and the stretching and speed strategy by mode is implemented.

[0099] No problem of submerged nozzle sleeve eyes occurred in this heat, but a large number of slag inclusion defects appeared on the strip surface. The defect components scanned by the electron microscope are calcium aluminate. This shows that when the sulfur and oxygen in the molten steel are not controlled properly, a large number of endogenous inclusions will be generated in the molten steel, and LF cannot effectively desulfurize and remove inclusions. These inclusions will eventually break out during the strip rolling process and appear on the strip surface.

[0100] In addition, one or more technical solutions in the embodiments of the present application at least further have the following technical effects or advantages:

[0101] In the embodiments of the present application, a set of control standards for the cleanliness of molten steel in a multi-mode continuous casting and rolling production line is designed to provide qualified molten steel for the stability of continuous casting of the casting machine and the qualification rate of product quality.

[0102] In the embodiments of the present application, a stretching and speed strategy by mode for a multi-mode continuous casting and rolling production line is designed to ensure the smooth liquid level fluctuations under different modes and speeds, and reduce or eliminate product slag inclusion defects.

[0103] In the embodiments of the present application, by improving the cleanliness of molten steel and optimizing the stretching and speed strategy, the slag inclusion defects in the multi-mode continuous casting and rolling production line are reduced, the output of the casting machine is increased, and the product quality is improved.

[0104] The above are only the specific implementation manners of the present application, enabling those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for controlling slag inclusion defects on the surface of strip steel in a multi-mode continuous casting and rolling production line, the method comprising: Desulfurizing blast furnace molten iron to obtain desulfurized molten iron with a set sulfur content; The desulfurized molten iron is subjected to converter smelting to obtain converter molten steel with a set oxygen content; The converter molten steel is subjected to LF refining, and before the LF refining station is discharged, the molten steel is subjected to calcium treatment and soft blowing treatment for a set time to obtain refined molten steel with a set calcium content; as well as The refined molten steel is continuously cast, and in the speed increasing stage of the continuous casting, the casting machine speed is increased in a divided mode to control the crystallizer liquid level fluctuation range to be ±3 mm, so as to obtain a continuously cast billet.

2. The method according to claim 1, characterized in that: In terms of parts per million, the set sulfur content is ≤20 ppm.

3. The method according to claim 1, characterized in that The desulfurization treatment adopts a deep desulfurization mode, and the slag bright surface of the desulfurization treatment is greater than 90%.

4. The method according to claim 1, characterized in that: In terms of parts per million, the set oxygen content is 250 ppm to 500 ppm.

5. The method according to claim 1, characterized in that The setting time is 10 minutes to 12 minutes.

6. The method according to claim 1, characterized in that In terms of parts per million, the set calcium content is 20 ppm to 30 ppm.

7. The method according to claim 1, characterized in that In terms of mass fraction, the sum of the contents of FeO+MnO in the top slag of LF refining is less than 1%, and the ratio of Als / Alt in the top slag of LF refining is greater than 0.

91.

8. The method according to claim 1, characterized in that The sub-mode casting speed is improved to meet the following relationship: During the pouring phase, the speed is increased to 3.5 m / min at a step of 1.5 m / min without maintaining, and the single-brick mode is implemented; Keep a steady state for 3 minutes at a step of 0.2 m / min, increase the speed to 4.3 m / min, and execute the semi-headless mode; Keep a stable state for 3 minutes at a step of 0.2m / min, increase the speed to the target pulling speed, and execute the headless mode.

9. The method according to claim 1, characterized in that: The step of continuously casting the refined molten steel comprises: The refined molten steel is injected from the ladle into the tundish; the tundish is protected by an alkaline covering agent, and the basicity of the covering agent is ≥2; The refined molten steel is injected from the tundish into a crystallizer for continuous casting.

10. The method according to claim 9, characterized in that The surface of the refined molten steel in the crystallizer is covered with protective slag, the basicity of the protective slag is 0.95-1.36, and the viscosity of the protective slag at 1300° C. is 0.7 Pa·S-1.0 Pa·S.

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